For their 2013 paper ‘Is everything we eat associated with cancer? A systematic cookbook review‘, Jonathan D Schoenfeld and John PA Ioannidis selected recipes at random from a popular cookbook and evaluated all the ingredients to see if there was any research literature suggesting they increased or decreased cancer risk.
Interestingly, they found that most ingredients can causes cancer and cures it – at the same time! Based on these findings, it seems that red wine will add years to your life one week but then kills you quicker the next…
However, the authors’ discussion summarized very well how chaotic the literature examined really is and provided an explanation for the above mentioned results:
80% of ingredients from randomly selected recipes had been studied in relation to malignancy and the large majority of these studies were interpreted by their authors as offering evidence for increased or decreased risk of cancer. However, the vast majority of these claims were based on weak statistical evidence. Many statistically insignificant “negative” and weak results were relegated to the full text rather than to the study abstract. Individual studies reported larger effect sizes than did the meta-analyses.
The statistical support of the effects was weak (0.001 < P < 0.05) or even nonnominally significant (P > 0.05) in 80% of the studies. It was also weak or nonnominally significant, even in 75% of the studies that claimed an increased risk and in 76% of the studies that claimed a decreased risk
In other words, there are lots of studies out there that claim to find a link, either for increased risk or a protective effect, between this food or that ingredient and cancer, but very few of them actually provide convincing support for their hypothesis.